This invention relates to a persistent superconducting switch for use with conduction-cooled and liquid cryogen boiling-cooled superconducting magnets. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter called "MRI").
As is well known, a magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid heIium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that after a power source is initially connected to the coil to introduce a current flow into the coils, is removed the current will continue to flow through the coils due to the negligible resistance, thereby maintaining a magnetic field.
To maintain current flow in the magnet coils after removal of power it is necessary to complete the electric circuit within the cryogenic environment with a superconducting switch which is connected in parallel with the power supply and the magnet coils. The superconducting switch consists of a superconducting conductor which when driven into the non-superconducting or normal state has sufficient resistance so that current from the power supply will essentially all flow through the magnet coils during "ramp-up." When the desired magnetic field current is achieved the switch is returned to its superconducting state, and the magnet current commutates out of the power supply and through the switch when the power supply is ramped down. The magnet is now in "persistent mode".
There are four key characteristics that a superconducting switch must exhibit. One, it must be capable of easily and quickly being transformed (switched) from the superconducting state to the normal state and vice versa. There are three ways to do that: a) thermally--by heating .the superconducting material above its transition temperature; b) magnetically--by applying a magnetic field greater than the critical field of the material; or c) electrically--by raising the current in the material above its critical current. The thermal method is the most common. Two, it must have a high enough resistance in its normal state such that current flow through the switch during ramp is negligible so that excessive heating in the cryogen environment is not produced. Three, the switch must be stable. That is, it must not transition from the superconducting to normal state; and it cannot burn out if it does. Four, it must be capable of carrying the same high currents as the magnet coils. One of the challenges in designing a superconducting switch is to balance the conflicting requirements of stability against high normal resistance and low heat input.
Most superconducting magnets, and superconducting switches if incorporated, have utilized NbTi as the superconducting material and have been cooled by direct contact with a boiling bath of flowing liquid helium (.about.4.2K). While the use of liquid helium to provide cryogenic temperatures has been widely practiced and is satisfactory, helium is found and is commercially obtained only in the state of Texas. As a result, providing a steady supply of liquid helium to superconducting magnet installations all over the world, particularly MRI systems, has proved to be difficult and costly. Consequently, considerable research and development effort has been directed at eliminating the need for a boiling cryogen such as liquid helium by developing superconducting materials and magnets which are superconducting at relatively higher temperatures such as 10 degrees Kelvin (10K). Such materials could then be cooled by conduction through the use of standard, commercial two-stage cryocoolers which alternately compress and expand gaseous helium in a closed system to remove heat from a given area. Nb.sub.3 Sn and other higher temperature superconducting materials (such as YBCO) can be made superconducting at such relatively high temperatures.
However, the use of Nb.sub.3 Sn in superconducting magnet applications has been limited to very high field applications, beyond the capability of NbTi. This has been mainly due to the fact that it is a brittle material. However, in addition to the ability of Nb.sub.3 Sn to generate higher magnetic fields than NbTi, it also remains superconducting at higher temperatures {Tc(Nb.sub.3 Sn) =.about.18K; Tc (NbTi)=.about.9K}.
All superconductors must incorporate stabilizing material to provide an alternate path for current flow in case of a fault condition in which the superconducting material quenches; that is, reverts from its essentially non-resistive state to its "normal" or resistive state. Because Nb.sub.3 Sn is brittle, stabilizing materials, particularly high resistance materials (e.g., brass) required for a switch, can subject Nb.sub.3 Sn to severe compressional strains due to differences in thermal contraction which can severely degrade its performance (critical current). Those strains result from both cool down to room temperature from the fabrication temperature and subsequent cool down from ambient to the superconducting operating temperature of less than 18K. That is, the ability of Nb.sub.3 Sn to carry superconducting currents is strongly dependent on any strain to which it is subjected. As a result, thermal strains should be avoided.
The Nb.sub.3 Sn and stabilizing material can be assembled into a composite conductor in two forms--one is multifilaments of Nb.sub.3 Sn in a wire-formed stabilizer; the other is a Nb.sub.3 Sn tape (typically less than 0.1 mm thick) with stabilizer bonded (soldered) to both sides. However, tape conductors are especially vulnerable to a phenomenon called flux jumping. This is a quick motion of the magnet flux penetrating a superconductor which generates local heating that may be sufficient to cause a quench. The incorporation of stabilizer in the conductor, in addition to providing an alternate current path when a superconductor quenches, can also reduce flux jumping and limits temperature excursions if it does occur, thereby preventing a quench. However, stabilizing materials for a superconducting switch must have relatively high electrical resistance, but the poor thermal conductivity associated with that high resistivity does not provide effective stabilization against flux jumps.
Flux jumping has been described as a kind of electromagnetic thermal instability affecting all high-field superconductors. Flux jumping or heating from other sources when current is flowing through a superconducting switch must be minimized to avoid quenching and, therefore, discontinuance of superconducting magnet operation. For a standard helium pool--boiled, cooled magnet, for example and an MRI system, a quench of the superconducting magnet causes a major disruption of equipment use because it results in a shut down and requires a time-consuming and expensive (if replenishment of the liquid helium cryogen is required) subsequent ramping up of the superconducting magnet. Heating is a particularly difficult problem in a conduction-controlled superconducting switch, because of the limited cooling capacity as compared with the use of a boiling cryogen such as liquid helium.
Thus, there are a number of conflicting characteristics and properties required of material and structure for stable superconducting switches. The problem of differential expansion of materials, or thermal contraction, is of particular concern and must be overcome in a stabilizer which is laminated together with the superconductor in an oven at temperatures around 600K, but in operation is cooled to, and operated in, the range of 4-10K. The thermal contraction rate of the stabilizer should be as close to the superconductor as possible in order to minimize the thermal stress to the superconductor, and the stabilizer should have good thermal conductivity to conduct heat away from the superconductor which is generated because of current flow and flux motion in the superconductor. In addition, with the high currents existing during superconductivity, within the requirements which will satisfy the superconducting magnet ramp-up rate and size, the electrical conductivity of the stabilizer should be chosen as large as possible to further increase superconductor stability.
Various approaches to resisting flux jumping have involved slowing down the motion of the magnetic flux, and improving the cooling of the superconducting magnet. Other approaches have involved interleaving copper foils between the tape conductor and exposing the edges of the superconducting coil stack to boiling liquid helium. They have not proven to be completely satisfactory in meeting the overlapping and conflicting requirements which must be satisfied for a practical and satisfactory superconducting switch.